Image sensors are used in a wide variety of applications, for example, digital cameras, cellular phones, security cameras, optical mice, as well as various other medical, automobile, military, or other applications. Interest in image sensors with three-dimensional (“3D”) imaging capability is increasing as the popularity of 3D applications continues to grow in applications such as imaging, movies, games, computers, user interfaces, and the like. A typical passive way to create 3D images is to use multiple cameras to capture stereo or multiple images. Using the stereo images, objects in the images can be triangulated to create the 3D image. One disadvantage with this triangulation technique is that it is difficult to create 3D images using small devices because there must be a minimum separation distance between each camera in order to create the three dimensional images. In addition, this technique is complex and therefore requires significant computer processing power in order to create the 3D images in real time.
Some applications (e.g. gesture recognition and gaming) benefit from the acquisition of both two-dimensional (2D) images and 3D images. Using the triangulation technique to acquire 3D images and combining the 3D images with 2D images also requires significant processing power. To acquire 3D images in real time, active depth imaging systems based on optical time of flight measurement are sometimes utilized. These time of flight systems typically employ a light source that directs light at an object, a sensor that detects the light that is reflected from the object, and a processing unit that calculates the distance to the object based on the round trip time that it takes for light to travel to and from an object. Some known time of flight sensors attempt to capture 2D images with the same photodetectors that capture the 3D time of flight images by sharing the frame time of the photodetectors in the sensor between 2D imaging and 3D time of flight imaging. However, this requires relatively high speed circuits and may result in poor signal for both 2D and 3D image capture due to low exposure durations.